1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device and a method of manufacturing the same, and more particularly, to an EL device with low manufacturing costs and improved yield due to simplified structure and use of an organic light emitting transistor (OLET) and a method of manufacturing the same.
2. Description of the Related Technology
Flat display devices such as liquid crystal devices, EL devices, or in EL devices are classified into passive matrix (PM) and active matrix (AM) according to their driving methods. In passive matrix flat display devices, anodes and cathodes are arranged in columns and rows, respectively. A row driving circuit supplies a scanning signal to one of the cathodes to select only one row and a column driving circuit supplies a data signal to each pixel of the selected row. Because there are no transistor-capacitor circuits for the pixels, the image is not stored between refresh cycles and the image is less bright and sharp than an active matrix display.
On the other hand, an active matrix flat display device controls a signal input to each pixel using thin film transistors and is suitable for displaying a plurality of images. Accordingly, active matrix display devices are widely used as display devices for displaying a moving picture.
The thin film transistors of active matrix flat display devices each have source and drain regions in which impurities are heavily doped, a semiconductor layer having a channel region between the source and drain regions, a gate electrode which is insulated from the semiconductor layer and is located above the channel region, and source and drain electrodes contacting the source and drain regions, respectively.
The semiconductor layer is often composed of amorphous silicon or poly silicon. Amorphous silicon can be deposited at low temperature, but has electric characteristics that is deteriorated and thus is unreliable and can not easily realize a large-area flat display device. Accordingly, poly silicon is widely used. Since poly silicon has high current mobility of several tens through several hundreds of cm2/V·s, excellent high-frequency operation characteristics, and a low leakage current, it is suitable for a high-precision and large-area flat display device.
However, if the semiconductor layer is made of poly silicon, a process for crystallizing amorphous silicon is required. The crystallizing process must be generally performed at a high temperature of 300° C. In this case, a substrate composed of, for example, a plastic material for realizing a flexible display device can not be used. Accordingly, research into an organic thin film transistor which does not require the crystallizing process has increased.
Flat display devices require high operating speeds and uniform quality in order to process a high quality moving picture. Particularly, in the case of an EL device, a circuit having excellent characteristics must be realized in order to display the moving picture with high quality. That is, in an EL device, a driving element (transistor) is separately provided to each pixel in addition to a switching element (transistor) for switching each pixel. Also, a plurality of thin film transistors may further be included in various compensating circuits associated with the display.
If the EL device has a display unit connected to the circuit having the above-mentioned complicated structure, the manufacturing process thereof is complicated and, the yield thereof is decreased, thus resulting in increased manufacturing costs.